What is your preferred format for music collecting?

CD's mainly. I like collecting vinyl too ... but as far as sound quality goes(which should be the most important factor), it's CD's for me.

This. CD is the superior format to both vinyl and cassette, though there are a handful of exceptions (mostly older music) where the vinyl or cassette master is the better choice. I currently dont have a cassette player, and unfortunately im still in the hunt for a new pre-amp for my record player, so lately CD reigns supreme for me.

Though what I usually do is rip my CDs in lossless format via dbpoweramp and keep the CD pristine for hard copy purposes, so I guess you could actually say my preferred format is FLAC/ALAC via bit-perfect playback.
 
CD first, but have semi-recently fell for the vinyl bug. I'm working my way through the 80s era trying to obtain a lot of the old classic stuff. I'll buy select new albums on vinyl depending on band/price.

I also still have my 1,000 or so vintage (gotta love that word...) cassettes I still pop in from time to time. They definitely have a nostalgic sound to them :)